The invention relates to a process of the type designated in the preamble of patent claim 1. The invention also relates to a device for implementation of this process.
Laser-ultrasound systems have already been successfully employed in measuring wall thickness during hot-rolling. This can be seen, for example, from the publication by R. Keck, B. Kruger, G. Coen, and W. Hasing, "Wall Thickness Measurement on 1230.degree. C. Hot Tube Loops with a New Laser-Ultrasound System", Steel and Iron, 1987, pp. 1057 to 1060. This system provides for the application of a laser interferometer in which the surface motion of the test material induces a frequency modulation in the radiation of the illumination laser and the frequency and amplitude of the ultrasonic wave are contained. For demodulation, a portion of the scattered-back light is directed by a convex lens to a (velocity-modulated) interferometer, in which the received light is broken up by a beam-splitter and is brought into interference after passing through paths of varying lengths. In the process, portions of wave trains that have left the laser at different times interfere with each other. Given a constant frequency of the received laser light, the phase allocation in the interferometer remains the same, which results in a constant brightness in the detector. Due to the Doppler effect, the ultrasound causes the laser light to undergo a slight change in frequency, which results in a time lag at the point of interference. When the phases do not overlap in their original allocation, the modified phase allocations produce differing brightness values in the detector. The frequency difference caused by the ultrasound in the interfering waves is a measure of frequency and amplitude of the ultrasonic wave.
There is maximum of phase shift in the interference of those waves which are modulated in opposite directions by maximal surface velocities. There is maximum velocity in the crossovers of the ultrasonic waves. Ultrasonic waves in which the time interval between crossovers corresponds to the phase velocity difference in the arms of the interferometer are those most sensitively registered. In order to assure the highest degree of sensitivity, the apparatus requires a very expensive secondary regulating device to evaluate the arising signals in satisfactory fashion. Fluctuations in intensity due to rough surfaces in the test material or to an oblique angle of incidence of the laser light, precisely as occurs under operating conditions, must be eliminated.
Based on the above, there is a need to provide at reduced expense an evaluation of the phase allocation for the demodulation of the reflected laser light. It should be possible to use any desired portion of the brightness progression caused by the difference in two wave trains during phase displacement (which brightness progression forms the characteristic sinusoidal demodulation curve), thereby eliminating the restriction to only the steepest part of a sine curve arm at a 90.degree. or 270.degree. phase displacement (FIG. 4).